Method and device for the interference elimination of a redundant acoustic signal

ABSTRACT

A method and device are provided for the interference elimination of a redundant acoustic input signal of an acoustic reproducing device, such as a mobile phone or a hearing aid, according to which the interference is concentrated in a partial frequency range of a total frequency range of the input signal. The present invention seeks to improve the sound quality of an acoustic output signal produced. The inventive method includes the following subsequent steps: a) removing the partial frequency range of the input signal, in which the interference is concentrated, b) dividing up the intensity of the frequency range of the input signal that was allowed to pass in the first step into an input signal portion to be maintained and an input signal portion to be further processed, c) synthesizing the partial frequency range of the input signal removed in step a) in accordance with the input signal portion to be further processed, and d) combining the input signal portion from step b) to be maintained and the synthesized input signal portion from step c), thereby producing an output signal that is interference-eliminated as compared to the input signal.

BACKGROUND OF THE INVENTION

The present invention relates to a method for removing interference froma redundant acoustic input signal to an acoustic reproduction apparatus,such as a mobile telephone or a hearing aid, in which the interferenceis concentrated in one frequency band element of an overall frequencyband of the input signal, and further relates to an apparatus forcarrying out such method.

When acoustic signals, such as speech signals, are being received innoisy environments, such as in a motor vehicle, in an industrialenvironment in which there is machine noise, or in public areas, theproblem arises of removing interference from the speech signal before itis reproduced by an acoustic reproduction device. In this case, the aimis to reduce as much as possible undesirable interference signalcomponents in the input signal. A further aim is to prevent artifactsfrom occurring, including those which affect the interference signal.

It is known to use so-called gradient microphones in order to removeinterference from acoustic signals. Such microphones have a directionalcharacteristic which is used to attenuate interference signal componentsfrom directions other than the main direction of the microphone, incomparison to a wanted signal in the main microphone direction. However,these microphones have the disadvantage that they are more sensitive towind and breathing noise. This sensitivity can be improved by the use ofwind protection, but this is so large that it cannot be used with alltypes of acoustic reproduction devices. For example, the use of windprotection is not typically used for a mobile telephone or a hearing aidas the acoustic reproduction devices.

Gradient microphones are normally implemented in hearing aids on thebasis of two omnidirectional microphones, using a so-called “delay andsubtract” approach. However, the directional effect of this approach,particularly in the lower frequency band of the input signal, is verysensitive to mismatching of the microphone characteristics, which cannotbe avoided in practice. Furthermore, there are difficulties with respectto the internal microphone noise.

It is also known for statistical interference noise suppression methodsto be used for suppression of interference signals in acoustic signals,such as spectral subtraction methods, the Wiener filter, and methodsfrom Ephraim & Malah, R. Martin, etc. These methods make use of thedifferent statistical characteristics of desired speech signals and ofinterference signals. It is assumed that an interference signal isconsiderably more stationary than a desired speech signal. Theinterference signal component is generally estimated during pauses inspeech, and is continuously subtracted on a frequency-specific basisfrom the input signal that is subject to interference. The estimation ofthe interference component is particularly difficult in the case ofnon-stationary interference, particularly when the interference signalis at a higher level than the desired signal.

Artifacts occur during the subtraction of the estimated noise orinterference component, and are normally referred to as “musical tones.”Oftentimes, however, the remaining residual interference signal aftercarrying out the interference noise suppression method no longer has theoriginal interference signal character (that is to say, the noise soundsdifferent), which many users of acoustic reproduction devices are notused to and find disturbing. If the signal-to-noise ratio is low, forexample <5 dB, the subtraction principle generally fails, so that theinput signal that is subject to interference is generally assessed bythe user as sounding better than the processed signal which resultsafter carrying out the interference noise suppression method.

Against this background, the present invention is directed toward amethod for removing interference from a redundant acoustic signal in anacoustic reproduction apparatus, and to an apparatus for carrying outsuch method. Pursuant to such method and apparatus, the sound quality ofthe resultant acoustic output signal is improved.

SUMMARY OF THE INVENTION

Accordingly, a method is provided for removing interference from aredundant acoustic input signal to an acoustic reproduction apparatus,such as a mobile telephone or a hearing aid, in which the interferenceis concentrated in one frequency band element of an overall frequencyband of the input signal.

The method includes the following successive steps:

-   -   a) removal from the input signal of the frequency band element        in which the interference is concentrated;    -   b) splitting of the intensity of the frequency band of the input        signal which is passed through in step a) into an input signal        element to be retained and an input signal element to be        processed further;    -   c) synthesis of the frequency band element of the input signal        removed in step a) on the basis of the input signal element to        be processed further; and    -   d) combination of the input signal element to be retained from        step b) and the synthesized input signal element from step c) in        order to produce an output signal with less interference than        the input signal.

One major difference between the method of the present invention and themethod according to the prior art is that the frequency band element ofthe input signal in which interference is concentrated can be completelyrejected. In contrast, the prior art has always adopted the approach ofprocessing the relevant frequency band element so as to minimizeinterference.

According to the inventive method, the frequency band element which isremoved from the input signal and in which the interference isconcentrated is synthesized; in particular, on the basis of a frequencyband which has not been removed from the input signal. At this point,use is made of the fact that the input signal element which has not beenremoved and is intended to be processed further allows conclusions to bedrawn about the frequency profile of the frequency band element that hasbeen removed, owing to the redundancy in the acoustic input signal. Forexample, it can be assumed that an associated frequency spectrum existsfor every acoustic sound, in which case a frequency spectrum element canbe used to synthesize a spectrum that is adjacent to that frequencyspectrum element.

Also, according to the inventive method, the input signal element to beretained, that is to say that part of the overall frequency band of theinput signal which is not significantly subject to interference, iscombined with the synthesized frequency band element in which theinterference does occur, in order to produce an output signal with lessinterference than the input signal.

The synthesis of the frequency band element of the input signal in stepc) is preferably carried out via a bandwidth widening method. Forexample, in order to widen the bandwidth in the lower frequency band(<300 Hz), it is known from EP 0 994 464 A for signal components in thelower frequency band of a speech signal, whose low frequencies arelimited by a high-pass filter, to be reproduced, with the describedhigh-pass filtering being carried out, for example, at the remotesubscriber when speech is being transmitted via telephone.

The reproduction is, in this case, carried out by generation offrequencies in the lower frequency band via non-linear signalprocessing, according to which sub-harmonic frequencies of the signalare produced and are added to the high-pass signal. It should bestressed that the method described in EP 0 994 464 A is not based on theremoval of interference from a signal arriving at a mobile telephone ora hearing aid. In contrast, the novel method may be used, inter alia,for mobile telephones, hands-free devices, hearing aids and othercommunication terminals which are intended for mobile use or use inenvironments that are subject to interference.

The synthesized frequency band element of the input signal is preferablyfiltered in step c) for matching to the input signal element of theinput signal to be retained. This ensures that the combination process(which is carried out in d)) of these two signal elements leads to anoutput signal with reduced interference, whose frequency spectrumcorresponds as well as possible to the overall frequency spectrum of thewanted signal in the input signal, particularly with regard to theintensity. The characteristics of the spectral filter that is used instep a) also must, of course, be taken into account during the synthesisof that frequency band element of the input signal which is removed orfiltered out in step a) because it defines the frequency band which isto be synthesized. Special effects occur for the edge area of a filterthat is used.

For many environmental characteristics, such as of a mobile telephone ora hearing aid, it is advantageous for the frequency band element of theinput signal which is synthesized in step c) to be at the low-frequencyend of the overall frequency band of the input signal. For example,interference in a motor vehicle or so-called pop-and-flow noise causedby wind and breathing typically occurs in the low-frequency region ofthe input signal. The intensities of the interference frequently areconsiderably higher than the wanted signal intensities. If attempts aremade to overcome this interference using the above-mentioned spectralsubtraction method from the prior art, this can be done only by theacceptance of artifacts, whose intensity is high. The method of thepresent invention is not subject to these restrictions.

Likewise, it is possible to use the inventive method to reduce orprevent interference which is at the high-frequency end of the overallfrequency band of the input signal. Even interference which isconcentrated in an intermediate frequency band of the overall frequencyband of the input signal can be suppressed.

A further advantage of the inventive method is that even highlynon-stationary interference can be removed, which is impossible usingspectral subtraction methods. The inventive method also can be combinedwith a gradient microphone, since its disadvantages, such aspop-and-flow noise sensitivity, can be compensated for.

The above-mentioned object is achieved with regard to an apparatus forcarrying out the method of the present invention by an apparatus forreproduction of or passing on a redundant acoustic input signal, such asa mobile telephone or a hearing aid, which has interference suppressioncapabilities for suppression of input signal interference which isconcentrated in a frequency band element of the overall frequency bandof the input signal. The parts for interference suppression include:

-   -   a spectral filter for removal of the frequency band element of        the input signal in which the interference is concentrated;    -   a splitting device, which is arranged downstream from the        filter, for splitting the intensity of the frequency band of the        input signal that is passed through into an input signal element        to be retained and an input signal element to be processed        further;    -   a processing device, which is arranged downstream from the        splitting device, and to which the signal element of the input        signal to be processed further is supplied, for synthesis of the        filtered-out frequency band element of the input signal on the        basis of the input signal element to be processed further; and    -   an output signal production device, to which the input signal        element to be retained is supplied from the splitting device,        and to which the synthesized input signal element is supplied        from the processing device, for combination of the input signal        element to be retained and of the synthesized input signal        element in order to produce an output signal with less        interference than the input signal.

The method of operation of this apparatus is, in fact, evident from theinventive method discussed above, as the apparatus provides the devicesthat are required for the individual method steps.

It is preferable for the apparatus to have a device for detectingwhether the input signal has interference in the cut-off band of thefilter, and for a switch to be provided for bridging the interferencesuppression parts if there is no interference in the cut-off band of thefilter. This ensures that the interference suppression parts areswitched on only when interference is actually present in the cut-offband of the filter.

It is also advantageous for the apparatus to have a device for detectionor estimation of the intensity of any interference, and to have a switchfor bridging the interference suppression parts on the basis of theintensity detection/estimation. This makes it possible for theinterference suppression parts to be switched on only when this is foundto be necessary on the basis of the intensity of the interference. Inthis case, it is also possible to manually set a threshold value for thedetection or estimation of the intensity of any interference; forexample, by the user of a mobile telephone. Instead of the switch forbridging the interference suppression parts, a device for overlaying theinput signal element in the cut-off band of the filter and of thesynthesized signal element, as a function of the amount of interferencein the cut-off band of the filter, also may be provided.

With regard to both the method and the apparatus, it should be stressedthat any interference may, in principle, be concentrated in any givenfrequency band element of an overall frequency band of an acousticsignal. The respectively removed or filtered-out frequency band elementis then preferably synthesized via a bandwidth widening method, and iscombined with the retained frequency band, which was not filtered out,in order to produce an output signal with reduced interference.

Additional features and advantages of the present invention aredescribed in, and will be apparent from, the following DetailedDescription of the Invention and the Figures.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 shows a block diagram of an apparatus for removing interferencefrom a redundant acoustic input signal.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a method procedure and components required for carrying outthe method in order to improve the sound of a redundant acoustic inputsignal 1 that is subject to interference, such as a speech signal.Speech signals have the intrinsic feature that the frequency spectrumthat is present at any specific time is not all required to make astatement on what sound should be associated with the current frequencyspectrum. To this extent, speech signals are redundant; that is, some ofthe frequency spectrum can be dispensed with, particularly without anyloss of information about the overall frequency spectrum. Such conceptis based on the fact that the frequency spectrum element which has beendispensed with can be deduced from the retained frequency spectrum.

In detail and as can be seen in FIG. 1, the input signal 1 (after it hasbeen received by a receiving module, for example, a mobile telephone orhearing aid) is passed to a spectral high-pass filter 2 which has adefined filter characteristic which, in the present exemplaryembodiment, allows frequencies above 300 Hz to be passed with virtuallyno attenuation, while frequencies below 300 Hz are highly attenuated.The value of 300 Hz thus forms the cut-off frequency fA of the high-passfilter 2.

The signal in the frequency band that is passed through is passed fromthe high-pass filter 2 to a junction point 3 as a splitting device, inwhich it is split into a signal element 4 to be retained and a signalelement 5 to be processed further.

Neither the signal 4 to be retained nor the signal 5 to be processedfurther has any significant frequency component in the region below 300Hz, so that any interference which is concentrated below 300 Hz isremoved from the input signal 1. To this extent, the explained exemplaryembodiment relates to interference which is concentrated at thelow-frequency end of the overall frequency band of the input signal 1.

The next method step now includes the synthesis of the frequencyspectrum element removed by the high-pass filter 2, to be precise withthe aid of the frequency spectrum, which has been passed through, of thesignal 5 to be processed further.

The signal 5 to be processed further is first of all passed to alow-pass filter 6, which in the illustrated exemplary embodiment highlyattenuates frequencies above 2 kHz. The low-pass filter 6 is notabsolutely essential for carrying out the inventive method, butsimplifies signal further-processing in a non-linear processing device7. In particular, the use of the non-linear processing device 7 canresult in so-called intermodulation effects occurring, whose appearanceis reduced or avoided by the low-pass filter 6.

The non-linear processing device 7 now produces a low-frequency spectrumeither directly from the signal 5 to be processed further or from anoutput signal from the low-pass filter 6; to be precise, taking intoaccount the characteristic of the high-pass filter 2, which is known inadvance and is taken into account directly in the non-linear signalprocessing. The non-linear processing device 7 is used to reproduceamplitudes at the fundamental frequency and/or missing harmonics as afunction of the basic speech signal.

The signal which leaves the non-linear processing device 7 thus ideallyincludes the fundamental frequency and the missing harmonics of theinput signal 1, which have been removed in the high-pass filter 2,together with the interference.

The output signal in the form of a synthesized speech signal from thenon-linear processing device 7 is then passed to a bandpass filter 8,which passes frequencies below the cut-off frequency fA of 300 Hz of thehigh-pass filter 2 and a low cut-off frequency fB of, for example, 100Hz. In principle, the bandpass filter 8 also may be replaced by alow-pass filter with a cut-off frequency of 300 Hz. However, if it isintended to remove DC voltage components from the output signal from thenon-linear processing device 7, suitable filtering should be carried outby the bandpass filter 8.

An output signal from the bandpass filter 8 is passed to a linking point9 as an output signal production device, at which it is added to thesignal element 4 to be retained in order to widen the bandwidth, and inorder to obtain an output signal 10 with reduced interference, whichideally would have all of the speech wanted signal frequencies of theinput signal 1 with an appropriate amplitude.

The apparatus, as illustrated in FIG. 1, for removing interference fromthe input signal 1 also may be modified such that interference which isat the high-frequency end of the frequency spectrum of the input signal1 is removed by a low-pass filter, in which case it then would benecessary to provide a non-linear processing unit which can carry out abandwidth widening process in the direction of high frequencies. In thecase of high-frequency interference, the low-pass filter 6 from FIG. 1would have to be replaced by a suitable high-pass filter. The bandpassfilter 8 also would have to be replaced by a filter which allowssuitable linking of the synthesized signal element and the signalelement 4 to be retained at the linking point 9.

The interference may also occur within the overall frequency band of theinput signal 1. In this case, either the frequency band of the inputsignal 1 at the high-frequency end of the interference or the frequencyband of the input signal 1 at the low-frequency end of the interferencecan be used to carry out a bandwidth widening method, with thenon-linear processing unit being used to close a frequency gap that isproduced by a bandpass filter instead of the high-pass filter 2.

In further embodiments of the present invention, which are notillustrated in FIG. 1, parts may be provided for switching on theapparatus according to FIG. 1 as a function of external parameters.Examples which may be mentioned include a device for detecting whetherthe input signal has interference in the cut-off band of the high-passfilter 2, or a device for detection or estimation of the intensity ofany interference. In both cases, the respective device is designed tooperate a switch which is arranged upstream of the high-pass filter 2.

It is particularly advantageous to provide a device for overlaying theinput signal 1 in the cut-off band of the respectively used filter withthe synthesized signal element, instead of a hard switch. The degree ofoverlaying may be dependent on the intensity of any interference that ispresent. A device such as this for controlling overlaying of the typementioned may be provided, for example, at the linking point 9, with thedevice being supplied with an estimated or measured value of theintensity of an interference signal as a control variable.

Although the present invention has been described with reference tospecific embodiments, those of skill in the art will recognize thatchanges may be made thereto without departing from the spirit and scopeof the present invention as set forth in the hereafter appended claims.

1-8. (canceled)
 9. A method for removing interference from a redundantacoustic input signal to an acoustic reproduction apparatus, wherein theinterference is concentrated in one frequency band element of an overallfrequency band of the input signal, the method comprising: a) removingfrom the input signal the frequency band element in which theinterference is concentrated; b) splitting an intensity of the frequencyband of the input signal which is passed through in a) into an inputsignal element to be retained and an input signal element to beprocessed further; c) synthesizing the frequency band element of theinput signal removed in a) based on the input signal element to beprocessed further; and d) combining the input signal element to beretained from b) and the synthesized input signal element from c) toproduce an output signal with less interference than the input signal.10. A method for removing interference from a redundant acoustic inputsignal to an acoustic reproduction apparatus as claimed in claim 9,wherein the synthesis of the frequency band element of the input signalin c) is carried out via a bandwidth widening method.
 11. A method forremoving interference from a redundant acoustic input signal to anacoustic reproduction apparatus as claimed in claim 9, wherein thesynthesized frequency band element of the input signal is filtered in c)for matching to the input signal element of the input signal to beretained.
 12. A method for removing interference from a redundantacoustic input signal to an acoustic reproduction apparatus as claimedin claim 9, wherein the frequency band element of the input signalsynthesized in c) is at a low-frequency end of the overall frequencyband of the input signal.
 13. A method for removing interference from aredundant acoustic input signal to an acoustic reproduction apparatus asclaimed in claim 9, wherein the acoustic reproduction apparatus is oneof a mobile telephone and a hearing aid.
 14. An apparatus for one ofreproducing and passing on a redundant acoustic input signal whichincludes interference suppression parts for suppressing input signalinterference which is concentrated in a frequency band element of anoverall frequency band of the input signal, the interference suppressionparts of the apparatus comprising: a spectral filter for removing thefrequency band element of the input signal in which the interference isconcentrated; a splitting device, arranged downstream from the filter,for splitting an intensity of the frequency band of the input signalpassed through the filter into an input signal element to be retainedand an input signal element to be processed further; a processingdevice, arranged downstream from the splitting device and to which thesignal element of the input signal to be processed further is supplied,for synthesizing the filtered-out frequency band element of the inputsignal based on the input signal element to be processed further; and anoutput signal production device, to which the input signal element to beretained is supplied from the splitting device and to which thesynthesized input signal element is supplied from the processing device,for combining the input signal element to be retained and thesynthesized input signal element so as to produce an output signal withless interference than the input signal.
 15. An apparatus as claimed inclaim 14, further comprising: a device for detecting whether the inputsignal has interference in a cut-off band of the filter; and a switchfor bridging the interference suppression parts if there is nointerference in the cut-off band of the filter.
 16. An apparatus asclaimed in claim 14, further comprising: a device for one of detectingand estimating an intensity of any interference; and a switch forbridging the interference suppression parts based on the intensity ofthe interference.
 17. An apparatus as claimed in claim 14, furthercomprising: a device for one of detecting and estimating an intensity ofany interference; and a device for overlaying the input signal elementin a cut-off band of the filter and of the synthesized input signalelement as a function of the intensity of the interference.
 18. Anapparatus as claimed in claim 14, wherein the apparatus is one of amobile telephone and a hearing aid.